Removal of corrosive water-soluble salts, particularly chlorides of sodium and potassium from crude oil is an important processing operation in refining of crude oils. The process of desalting usually involves addition of 1 to 20 weight percent wash water to the crude oil with mixing to form a water-in-crude oil emulsion. Then, the water-in-crude oil emulsion is subjected to electrostatic demulsification or hydrocyclone treatment. Under the influence of electrostatic or centrifugal fields the dispersed water droplets coalesce and the water-in-oil emulsion is demulsified. Thereafter, the water containing water soluble salts is separated from the crude oil.
Most heavy crude oils contain asphaltenes and naphthenic acids and tend to form stable high viscosity water-in-oil emulsions. Both, the stability and the high viscosity of the water-in-oil emulsions present difficulties in demulsification. Inefficient demulsification results in inefficient dewatering and desalting of the heavy crude oils. Large volumes of a rag layer tend to form in the separator vessels and result in substantial oil loss during dewatering and desalting processes. Current methods using centrifuges, hydrocyclones and electrostatic demulsifiers require large doses of demulsifier chemicals to destabilize the water-in-oil emulsions. High emulsion viscosity requires higher than normal operation temperatures and longer residence times for desalting and/or dewatering these water-in-oil emulsions in the separator vessels. Thus, there is a continuing need for improved cost-effective methods to dewater and desalt water-in-oil emulsions especially those formed from heavy crude oils. In particular there is a need to determine the viscosity of water in-oil emulsions formed from heavy crude oils using heavy oil composition data prior to desalting and dewatering operations so that the operating conditions of the separator vessels can be adjusted prior to dewatering and desalting. There is also a need for improved methods to form low viscosity water-in-oil emulsions for a variety of applications. Such applications include low viscosity lubricant fluids and low viscosity pusher fluids for improved oil recovery. The present invention addresses these needs. In particular, hydrocarbon oils that contain asphaltenes and naphthenic acids, such as crude oils, tend to form water-in-oil emulsions with a wide range of viscosity.